Transient blocking units (TBUs) have been in use for some time for protecting sensitive electrical devices and/or circuits from damage caused by electrical transients. An early reference relating to TBUs is U.S. Pat. No. 5,742,463 by Harris. FIG. 1 shows a simple example of a conventional TBU. In this example, two depletion mode (i.e., normally-on) transistors, Q1 and Q3, are connected in series such that the same current ITBU flows through Q1 and Q3. As ITBU increases, VDS of Q1 and VSD of Q3 both increase. The transistor types are selected such that as VDS of Q1 increases, the voltage applied to the gate of Q3 acts to shut off Q3. Similarly, as VSD of Q3 increases, the voltage applied to the gate of Q1 acts to shut off Q1. The positive feedback inherent in this arrangement leads to a rapid transition of the TBU from a normal low-impedance state to a high-impedance current blocking state once ITBU exceeds a predetermined threshold. In operation, a TBU can switch to its high-impedance state in response to an over-voltage or over-current condition from an electrical source, thereby protecting electrical devices or circuits connected to the TBU.
The TBU example of FIG. 1 is a unipolar (or uni-directional) TBU because it is only effective to block surges having a predetermined polarity (i.e., either positive surges or negative surges). FIG. 2 shows a conventional bipolar TBU. The circuit of FIG. 2 can be understood as providing two unipolar TBUs having opposite polarity in series. The first unipolar TBU is formed by the combination of Q1 and Q3, and the second unipolar TBU is formed by the combination of Q2 and Q3. FIG. 2 also shows a typical application for a TBU, where it is placed in series between an electrical source 202 and a load 204 to be protected.
TBUs have been investigated for both low voltage applications and high voltage applications. High voltage applications tend to require specialized TBU device and circuit approaches, e.g., as considered in US 2006/0098363 and US 2006/0261407. As another example, transistors fabricated with silicon carbide (SiC) can have increased breakdown voltage compared to Si transistors. However, SiC transistors are very costly to fabricate.
More recently, Gallium Nitride (GaN) material technology has been employed for high voltage device fabrication, e.g., as considered in U.S. Pat. No. 6,768,146. GaN has a large bandgap combined with high carrier mobility, making it an attractive material (compared to Si) for high-voltage and highly conductive devices. GaN has a significant advantage with respect to SiC because it can be deposited on non-native substrates relatively easily, thereby significantly reducing the cost of GaN devices compared to SiC devices. It is estimated that GaN devices may be up to 10× less expensive than comparable SiC devices.
However, GaN transistors exhibit a highly undesirable “current collapse” behavior, where the channel conductance of a GaN transistor decreases markedly after the device is exposed to high voltage at the source and/or drain. The conductance eventually recovers, although it can take a long time to do so (e.g., order of 10 s worst case). Current collapse is attributed to traps in the GaN material arising from substrate defects. Methods for reducing current collapse, either by reducing defects in the GaN, or in details of device design (e.g., as considered in U.S. Pat. No. 7,002,189) are under investigation. However, it is expected that GaN transistors will continue to exhibit current collapse for at least several years, and perhaps indefinitely.
This current collapse issue renders GaN transistors useless for most high voltage switching applications, despite the otherwise favorable cost and performance provided by GaN. In fact, elimination of the current collapse phenomenon in GaN transistors (by improved fabrication technology) is typically regarded by art workers as a prerequisite for the use of GaN transistors in commercial HV applications. Accordingly, it would be an advance in the art to provide a TBU suitable for use with high voltage transistors that can exhibit current collapse, such as GaN transistors.